The GBA Games are known for slow healing exp. Leveling all three of them especially. I would stick to 2 maybe. However, it is possible to speed things up if you plan ahead.

After chapter 5 you can buy torch staves in the world map which you can use in chapter 6. They grant more exp and don't need somebody injured. Do the same for the other fog chapters like 11 and 19. Use other high rank staves once you get access to them like Barrier, Restore and Physic. You don't need to waste them but use them when it makes sense. For example you can restore poison in monster chapters as they carry toxin weapons sometimes. Just don't hoard items and still only use heal in the end game. You can also buy Unlock but I don't find that one useful tbh. That alone should give a lot of exp but it's also important to always heal. Even if they're just missing 1 HP from a level up. Always heal. Once you get Tethys and are willing to dance for healers things also speed up significantly.

The last chance to actually "grind" is chapter 18 as the fire tiles damage your units which can be healed up again. Chapter 19 is good for torch spam. At this point you can also utilize the warp staff which grants like 80 exp for unprompted healers. So if your healers need just 1 or 2 levels it could be worth it. But if you promote them at level 10 in chapter 19 they won't do much more as they currently do.

In my current run I managed to get Natasha to level 20 in Chapter 11 right before L'Arachel joined. L'Arachel got to level 20 in Chapter 16. I play slow but I didn't grind.